How to fix a hole in the mesh
I don’t think there is a hole - it looks to be refraction showing the inside of the cup. Try reducing the IOR of your shader and see if the problem goes away - that will show whether there actually is a hole. Also, check your normals as that is critical with refractive surfaces since flipped normals will also flip the refraction properties. Also check that you have a realistic IOR for the material being modelled (eg, typically in the range 1.0 for air to around 1.33).